The present invention relates to an optical sensor head and an optical pickup for use in optical measurement, optical information processing and various other fields.
As higher output and more reliable semiconductor lasers and more accurate and lighter optical components have been developed in recent years, various optical sensors and optical disc devices such as compact discs and laser discs have come to be put to practical use for optical measurement and optical information processing with laser beams. A waveguide type optical head for a smaller and has lighter system has been proposed and drawn attention.
FIG. 3 shows the basic construction of the conventional optical head proposed as an optical pickup for optical discs. A semiconductor laser 1 is secured on a submount 2 made of, for example, copper. The submount 2 is fixed on a metal mount 3. Current flow between an Au wire 4 and the mount 3 allows the semiconductor laser 1 to emit laser beams. A thin film waveguide element 8, composed of a silicon substrate 5, a SiO.sub.2 buffer layer 6 formed by oxidation on the silicon substrate 5 and a glass waveguide layer 7 formed by sputtering on the buffer layer 6, is also fixed on the mount 3. On the waveguide layer 7 are formed a collimator lens 9, a beam splitter 10 and a focus grating coupler 11. Light beam emitted by the semiconductor laser 1 enters the waveguide layer 7 from one of the end faces thereof, is parallelized by the collimator lens 9 and concentrated by the focus grating coupler 11 as a spot 12 on a disc surface. Light reflected from the spot 12 returns through the focus grating coupler 11 into the waveguide and is redirected by the beam splitter 10 to an optical sensor 13 located on one side of the waveguide so that information recorded in the disc is read.
The collimator lens 9, the beam splitter 10 and the focus grating coupler 11 are susceptible to wavelength variation of the light source. The angle of reflection changes with the wavelength, causing the focal point 12 to deviate largely from the setting. Since an ordinary semiconductor laser emits laser beams of oscillation wavelength changing continuously or intermittently with the driving current and/or the room temperature, the proposed conventional optical head is not suitable for an optical pickup.
Furthermore, the light reflected from the end face of the waveguide (made up of glass, SiO.sub.2 and Si substrate) returns to the semiconductor laser, causing the oscillation wavelength of the semiconductor laser to change due to the following reason. That is, the laser beam reflected back from the waveguide end face to the semiconductor laser generates the composite resonator effect realized by the three reflection surfaces: the waveguide end face and the end faces of the semiconductor laser resonator, and the oscillation wavelength are selected by this composite resonator effect. However, since the wavelength to be selected changes with the room temperature and the driving current, the oscillation wavelength varies depending on conditions.
The wavelength selection in the conventional optical head with the mount made of copper or the like metal is described in detail with reference to FIG. 4.
FIG. 4 is a side view of the waveguide type optical head shown in FIG. 3. Assuming the lengths of the semiconductor laser 1 and the waveguide element 8 are 2l.sub.1 and 2l.sub.2, respectively, and the distance from the center of the semiconductor laser 1 to the center of the waveguide element 8 is l.sub.3, the distance L from the end of the semiconductor laser 1 to the end of the waveguide element 8 is calculated with the following equation: EQU L=l.sub.3 -(l.sub.1 +l.sub.2) (1)
The thermal expansion coefficient for the distance L is obtained from the following formula: ##EQU1## The term ##EQU2## (i=1, 3, 8) represents the thermal expansion coefficient of the semiconductor laser 1, the mount 3 or the waveguide element 8. For the proposed conventional head, the coefficient for the mount 3, ##EQU3## is 17.0.times.10.sup.-6, for it is made of copper, the coefficient for the semiconductor laser 1, ##EQU4## is 5.9.times.10.sup.-6, for it is made of GaAs-GaAlAs, and the coefficient for the waveguide element 8, ##EQU5## is 2.4.times.10.sup.-6, for it uses a Si substrate. Normally, l.sub.1 is 100 to 200 .mu.m, l.sub.2 is about 5 mm, and L is 5 to 50 .mu.m. Therefore, since the second and third terms take large values, the thermal expansion coefficient for the distance L as calculated by the expression (2) becomes very large when the room temperature changes. Minor room temperature variation results in a changed construction of the composite resonator, causing the oscillation wavelength of the semiconductor laser to vary.